Titan Pharmaceuticals Inc's experimental drug to treat opioid addiction was shown to be more effective than placebo in a clinical trial, but patients' response suggested that the proposed dosage might be too low, reviewers for the U.S. Food and Drug Administration said in documents published on Tuesday. The commentary was published on the FDA's website ahead of an advisory panel meeting to be held on Thursday and sent the company's stock down 42 percent to $1.19 in over-the counter trading. The reviewers said they would ask the advisory panel whether Titan should explore dosing further before the product is approved. The drug, Probuphine, is a long-acting version of buprenorphine, a drug sold by Britain's Reckitt-Benckiser Group Plc under the brand names Subutex and Suboxone. Unlike Subutex and Suboxone, which are dissolved daily under the tongue, Titan's drug is implanted under the skin of the upper arm during a 10-15 minute office procedure and delivers a continuous supply of buprenorphine for six months. "Overall, the response was not what one might hope for, given that the product ensures compliance with the medication for six months," the reviewers said. "It prompts speculation that the dose is simply not high enough." While the safety of Probuphine appears largely in keeping that of other buprenorphine products it presents an additional safety concern, reviewers said. It must be implanted, potentially leading to surgery-related complications. The reviewers said the product is in some ways similar to Norplant, an implantable, progestin-releasing contraceptive which is no longer marketed in the United States.